1. Field of the Invention
The present invention relates to a receiving apparatus suitable for a CDMA (Code Division Multiple Access) type cellular telephone system, a receiving method thereof, and a terminal unit for use with a radio system thereof.
2. Description of the Related Art
In recent years, a CDMA type cellular telephone system has become attractive. In the CDMA type cellular telephone system, a pseudo-random code is used as a spread code. A carrier of a transmission signal is spectrum-spread. The pattern and phase of each spread code in the code sequence are varied so as to perform a multiple access.
In the CDMA system, the spectrum spread method is used. In the spectrum spread system, when data is transmitted, the carrier is primarily modulated with the transmission data. In addition, the carrier that has been primarily modulated is multiplied by a PN (Pseudorandom Noise) code. Thus, the carrier is modulated with the PN code. An example of the primarily modulating method is the balanced QPSK modulating method. Since the PN code is a random code, when the carrier is modulated by the PN code, the frequency spectrum is widened.
When data is received, the received data is multiplied by the same PN code that has been modulated on the transmission side. When the same PN code is multiplied and the phase is matched, the received data is de-spread and thereby primarily modulated data is obtained. When the primarily modulated data is demodulated, the original data is obtained.
In the spectrum spread method, to de-spread the received signal, the same PN code that has been modulated on the transmission side is required for both the pattern and the phase. Thus, when the pattern and the phase of the PN code are varied, the multiple access can be performed. The method for varying the pattern and the phase of each spread code in the code sequence and thereby performing the multiple access is referred to as the CDMA method.
In cellular telephone systems, an FDMA (Frequency Division Multiple Access) system and a TDMA (Time Division Multiple Access) system have been used. However, the FDMA system and the TDMA system cannot deal with a drastic increase of the number of users.
In other words, in the FDMA system, the multiple access is performed on different frequency channels. In an analog cellular telephone system, the FDMA system is usually used.
However, in the FDMA system, since the frequency use efficiency is bad, a drastic increase of the number of users tends to cause channels to run short. When the intervals of channels are narrowed to accommodate the increase of the number of channels, the adjacent channels adversely interfere with each other and thereby the sound quality deteriorates.
In the TDMA system, the transmission data is compressed on the time base. Thus, the use time is divided and thereby the same frequency is shared. The TDMA system has been widely used as a digital cellular telephone system. In the TDMA system, the frequency use efficiency is improved in comparison with the simple FDMA system. However, in the TDMA system, the number of channels is restricted. Thus, it seems that as the number of users drastically increases, the number of channels runs short.
On the other hand, the CDMA system has excellent interference resistance. Thus, in the CDMA system, adjacent channels do not interfere with each other. Consequently, the frequency use efficiency improves and more channels can be obtained.
In the FDMA system and the TDMA system, signals tend to be affected by fading due to multipaths.
In other words, as shown in FIG. 9, a signal is sent from a base station 201 to a portable terminal unit 202 through a plurality of paths. In addition to a path P1 in which a radio wave of the base station 201 is directly sent to the portable terminal unit 202, there are a path P2, a path P3, and so forth. In the path P2, the radio wave of the base station 201 is reflected by a building 203A and sent to the portable terminal unit 202. In the path P3, the radio wave of the base station 201 is reflected by a building 203B and sent to the portable terminal unit 202.
The radio waves that are reflected by the buildings 203A and 203B and sent to the portable terminal unit 202 through the paths P2 and P3 are delayed from the radio wave that is directly sent from the base station 201 to the portable terminal unit 202 through the path P1. Thus, as shown in FIG. 9, signals S1, S2, and S3 reach the portable terminal unit 202 through the paths P1, P2, and P3 at different timings, respectively. When the signals S1, S2, and S3 through the paths P1, P2, and P3 interfere with each other, a fading takes place. In the FDMA system and the TDMA system, the multi-paths cause the signal to be affected by the fading.
On the other hand, in the CDMA system, with diversity RAKE method, the fading due to the multi-paths can be alleviated and the S/N ratio can be improved.
In the diversity RAKE system, as shown in FIG. 10, receivers 221A, 221B, and 221C that receive signals S1, S2, and S3 through the paths P1, P2, and P3 are disposed, respectively. A timing detector 222 detects codes received through the individual paths. The codes are set to the receivers 221A, 221B, 221C corresponding to the paths P1, P2, and P3, respectively. The receivers 221A, 221B, and 221C demodulate the signals received through the paths P1, P2, and P3. The received output signals of the receivers 221A, 221B, and 221C are combined by a combining circuit 223.
In the spectrum spread system, signals received through different paths are prevented from interfering with each other. The signals received through the paths P1, P2, and P3 are separately demodulated. When the demodulated output signals received through the respective paths are combined, the signal intensity becomes large and the S/N ratio improves. In addition, the influence of the fading due to the multi-paths can be alleviated.
In the above-described example, for simplicity, with the three receivers 221A, 221B, and 221C and the timing detector 222, the structure of the diversity RAKE system was shown. However, in reality, in a cellular telephone terminal unit of diversity RAKE type, as shown in FIG. 11, fingers 251A, 251B, and 251C, a searcher 251, and a data combiner 253 are disposed. The fingers 251A, 251B, and 251C obtain demodulated output signals for the respective paths. The searcher 252 detects signals through multi-paths. The combiner 253 combines the demodulated data for the respective paths.
In FIG. 11, a received signal as a spectrum spread signal that has been converted into an intermediate frequency is supplied to an input terminal 250. This signal is supplied to a semi-synchronous detecting circuit 255. The semi-synchronous detecting circuit 255 is composed of a multiplying circuit. The semi-synchronous detecting circuit 255 multiplies a signal received from the input terminal 250 by an output signal of a PLL synthesizer 256. An output signal of the PLL synthesizer 256 is controlled with an output signal of a frequency combiner 257. The semi-synchronous detecting circuit 255 performs a quadrature detection for the received signal.
An output signal of the semi-synchronous detecting circuit 255 is supplied to an A/D converter 254. The A/D converter 254 converts the input signal into a digital signal. At this point, the sampling frequency of a controller 258 is much higher than the frequency of the PN code that is spectrum-spread. In other words, the input signal of the A/D converter 254 is over-sampled.
An output signal of the controller 258 is supplied to the fingers 251A, 251B, and 251C. In addition, the output signal of the controller 258 is supplied to the searcher 252. The fingers 251A, 251B, and 251C de-spread the signals received through the respective paths, synchronize the signals, acquire the synchronization of the received signals, demodulate the data of these signals, and detect frequency errors of the signals.
The searcher 252 acquires the codes of the received signals and designates the codes of the paths to the fingers 251A, 251B, and 251C. In other words, the searcher 252 has a de-spreading circuit that multiplies a received signal by a PN code and de-spreads the signal. In addition, the searcher 252 shifts the phase of the PN code and obtains the correlation with the received code under the control of the controller 258. With the correlation between a designated code and a received code, a code for each path is determined.
An output signal of the searcher 252 is supplied to the controller 258. The controller 258 designates the phases of the PN codes for the fingers 251A, 251B, and 251C corresponding to the output signal of the searcher 252. The fingers 251A, 251B, and 251C de-spread the received signals and demodulate the received signals received through the respective phases corresponding to the designated phases of the PN codes.
The demodulated data is supplied from the fingers 251A, 251B, and 251C to the data combiner 253. The data combiner 253 combines the received signals received through the respective paths. The combined signal is obtained from an output terminal 259.
The fingers 251A, 251B, and 251C detect frequency errors. The frequency errors are supplied to the frequency combiner 257. With an output signal of the frequency combiner 257, the oscillation frequency of the PLL synthesizer 256 is controlled.
In the portable telephone terminal unit of the above-described RAKE type, the searcher 252 has the structure as shown in FIG. 13.
In FIG. 13, a digital signal is supplied from the A/D converter 254 (see FIG. 12) to an input terminal 301. As described above, the sampling frequency of the A/D converter 254 is higher than the frequency of a PN code. In other words, the digital signal is over-sampled. The digital signal is supplied from the input terminal 301 to a decimating circuit 302. The decimating circuit 302 decimates the signal received from the input terminal 301. An output signal of the decimating circuit 302 is supplied to a multiplying circuit 303.
A PN code generating circuit 304 generates a PN code that has spread on the transmission side. The phase of the PN code received from the PN code generating circuit 304 can be designated by a controller 258. The PN code received from the PN code generating circuit 304 is supplied to the multiplying circuit 303.
The multiplying circuit 303 multiplies the output signal of the decimating circuit 302 by the PN code received from the PN code generating circuit 304. Thus, the received signal from the input terminal 301 de-spreads. When the pattern and the phase of the received code match the pattern and the phase of the code received from the PN code generating circuit 304, the received signal de-spreads. Thus, the level of an output signal of the multiplying circuit 303 becomes large. The output signal of the multiplying circuit 303 is supplied to a level detecting circuit 307 through a band pass filter 306. The level detecting circuit 307 detects the level of the output signal of the multiplying circuit 303.
An output signal of the level detecting circuit 307 is supplied to an adding circuit 308. The adding circuit 308 cumulates output data a predetermined number of times (for example, 64 times). With the cumulated value of the output data of the level detecting circuit 307, correlation values of the code designated to the PN code generating circuit 304 and the received code are obtained. An output signal of the adding circuit 308 is supplied to a memory 309.
The phase of the PN code received from the PN code generating circuit 304 is shifted every predetermined number of chips. The correlation value is obtained from the output signal of the adding circuit 308 for each phase. The correlation value is stored in the memory 309. After the PN code has been designated for one period, the correlation values stored in the memory 309 are sorted in the order of larger correlation values by the controller 258. For example, three phases with the largest correlation values are selected. The selected phases are designated to the fingers 251A, 251B, and 251C (see FIG. 12).
FIG. 14 is a flow chart showing a process of the searcher. In FIG. 14, the phase of the PN code generating circuit 304 is designated to and initial value (at step ST101). The number of additions is cleared (at step ST102). The cumulated value of the adding circuit 308 is cleared (at step ST103).
When the initial phase is designated to the PN code generating circuit 304, the multiplying circuit 303 de-spreads the received signal with the designated PN code. The adding circuit 308 cumulates the de-spread signal level (at step ST104). Whenever the signal level is cumulated, the number of additions is incremented (at step ST105). Next, it is determined whether or not the number of additions exceeds a predetermined value (for example, 64) (at step ST106). The signal level is cumulated until the number of additions becomes 64. Thus, the correlation value is obtained. When the number of additions becomes 64, the correlation value at the time is stored in the memory 309 (at step ST107).
It is determined whether or not the last phase of the PN code generating circuit 304 has been designated (at step ST108). When the last phase has not been designated, the phase of the PN code is advanced or delayed by a predetermined value (at step ST109). Thereafter, the flow returns to step ST102. With the phase of the PN code that has been shifted for the predetermined value, the above-described process is repeated.
When the phase of the PN code is shifted for one period, at step ST108, the last phase of the PN code has been designated. Thus, the determined result at step ST108 becomes Yes. At this point, the correlation values stored in the memory 309 are sorted and the largest three correlation values are obtained (at step ST110). The three phases corresponding to the three largest correlation values are designated to the fingers 951A, 251B, and 251C (at step ST111).
As described above, in the searcher of a conventional CDMA portable telephone terminal unit of diversity RAKE type, while the phase of the PN code received from the PN code generating circuit 304 is being shifted, the correlation values of the received code and designated code are obtained. For example, three phases with the largest correlation values are designated to the fingers 251A, 251B, and 251C. Thus, when three paths are selected in the order of larger correlation values, the correlation values stored in the memory 309 are sorted and arranged in the order of larger correlation values.
Examples of the algorithm of the sorting process are quick sorting method and heap sorting method. However, in the algorithm of such a sorting process, a plurality of comparing calculations are required. Thus, the load applied to the controller 258 becomes large.